Infrared (IR) data transmission devices include one or more Light Emitting Diodes (LEDs) or Laser Diodes (LDs) (hereinafter commonly referred to as Communication Diodes (CDs)) for wireless optical data transmission purposes. Exemplary LEDs for IR data transmission devices include inter alia Model No. SFH 4000, commercially available from OSRAM Opto Semiconductors GmbH & Co. OHG Wernerwerkstrasse 2, D-93049 Regenburg Germany. Exemplary LDs for IR data transmission devices include inter alia Model No. MTE8087T, commercially available from Marktech Optoelectronics, 120 Broadway Menands, N.Y., 12204, U.S.A. CDs have an inherent typical forward voltage Vf in the region of 1.4V to 2.5V, depending on their type and operational conditions but suffer from a relatively large Vf tolerance in the region of ±15% due to manufacturing processes. CDs may be screened to meet a particular design specification but this time consuming approach is prohibitively expensive for certain equipment, for example, consumer electronic devices. Moreover, CDs have a temperature coefficient TCV of about −1.5 mV/K, such that a 10° C. temperature increase leads to an about 15 mV decrease in a CD's forward voltage Vf.
Mains and battery powered consumer electronic devices have undergone a major change in the last few years, and are now largely required to operate with low power supply voltages VCC of +5V, +3.3V and even +2.5V. Driver circuits for driving one or more parallel Communication Light Emitting Circuits (CLECs) typically include a single CD along a Communication Light Emitting Branch (CLEB) strapped between a power supply voltage VCC and GND in the case of +3.3V power supply, a pair of CDs along an CLEB in the case of +5V power supply, and possibly three CDs or more in the case of higher power supply voltages. Communication Diode Driver Circuits (CDDCs) are designed to illuminate CDs at about 90% of their maximum average LED drive current Imax hereinafter referred to as a nominal LED drive current IN so as not to shorten their lifetimes or cause malfunctions. However, power supply voltages can fluctuate by up to ±10%, which compounded with the variances of CDs' forward voltages Vf, and their inherent temperature dependency, can often lead to either insufficient or over-increased actual LED drive currents ILED(t). In the event that ILED(t)<IN, there is a resultant drop in CD light emission intensity thereby reducing the effective data transmission range, or in extreme circumstances precluding communication entirely. Against that, in the event that ILED(t)>IN for prolonged periods, a CDDC drives its CDs with an excessive LED drive current ILED(t), possibly shortening their lifetimes, or in extreme circumstances causing irreparable damage. Moreover, certain data transmission applications mandate relatively few or scarce digital data pulses arriving irregularly, thereby further complicating the design of a CDDC for accurately driving CDs.
One conventional approach for driving CDs includes the use of a so-called ballast resistor having a relatively large resistance, whereby the ballast resistor becomes the major device determining an actual LED drive current ILED(t) along an CLEB. Exemplary prior art references implementing this approach include inter alia GB 2 162 399 entitled LED modulator, U.S. Pat. No. 5,329,210 to Peterson et al., and U.S. Pat. No. 6,198,405 to Andersson et al. However, this approach typically requires a relatively high power supply voltage, and suffers from a poor overall device efficiency of 50% or even less due to considerable heat dissipation at the ballast resistor. Moreover, such heat dissipation can be disruptive to other nearby electronic devices rendering this technique unacceptable for certain applications.
Another conventional approach is to use Pulse Width Modulation (PWM) for controlling an actual LED drive current ILED(t) along an CLEB by changing pulse widths by means of a μcontroller core, timers, counters, pre-scalers, and the like. One exemplary PWM scheme is implemented in Microchip's PIC 16C781 commercially available from Microchip Technology, Inc., Christina Hecht, USA. Other PWM implementations are illustrated and described in GB 2 381 138 entitled ‘Driver circuit for light emitting devices’, U.S. Pat. No. 4,622,477 to Uda, U.S. Pat. No. 6,586,890 to Min et al., US Pub. No. 2003/0025465 to Swanson et al., US Pub. No. 2003/0122502 to Clauberg et al., US Pub. No. 2003/0041620 to D'Angelo et al., and an article entitled “A PWM modulator for wireless infrared communication”, by Koyuncu, Mesut et al., STW-2000 09 26-02:27, pages 351-353, Nov. 30-Dec. 1, 2000.
Other approaches for controlling LED drive currents are illustrated and described in inter alia JP 2003101123 entitled Semiconductor Laser Driver, US Patent Application Publication No. US 2003/0218585, and JP 63110685 entitled Drive Circuit of Light Emitting Element.